Barium titanate (BaTiO3) nanoparticles are synthesized under N-2 atmosphere by the hydrolysis and condensation of barium hydroxide octahydrate and titanium (IV) isopropoxide with controlled water addition. With increasing the amount of water addition, the primary nanoparticles tend to grow from 7.5 to 68.5 nm in average diameter and to agglomerate to form spherical secondary particles of 0.5-3 mu m in diameter. The influence of water content on the crystal size and structure was studied by X-ray diffraction via synchrotron radiation. As the amounts of added water increased, Rietveld analysis revealed that the lattice volume shrinks along the c-axis changing the crystal structure of BaTiO3 nanoparticles gradually from tetragonal to cubic. FT-IR spectra also indicated that the concentration of hydroxyl ions in BaTiO3 nanoparticles decreases with increasing water contents. It is thus concluded that adding sufficient amount of water is effective in reducing the concentration of protons in the oxygen sublattice during synthesis of BaTiO3 nanoparticles via alkoxide-hydroxide sol-precipitation process.